Charged nanofibers and methods for making

ABSTRACT

Described herein are nanofibers and methods for making nanofibers that include any one or more of (a) a non-homogeneous charge density; (b) a plurality of regions of high charge density; and/or (c) charged nanoparticles or chargeable nanoparticles. In one aspect, the present invention fulfills a need for filtration media that are capable of both high performance (e.g., removal of particle sizes between 0.1 and 0.5 μm) with a low pressure drop, however the invention is not limited in this regard.

CROSS-REFERENCE

This application is a Continuation of U.S. non-provisional applicationSer. No. 15/876,439 filed on Jan. 22, 2018, which is a Divisional ofU.S. non-provisional application Ser. No. 14/385,806 filed on Sep. 17,2014, which is a National Stage Entry of PCT/US13/31906 filed Mar. 15,2013, which claims the benefit of U.S. Provisional Application No.61/612,444, filed Mar. 19, 2012 and 61/717,163, filed Oct. 23, 2012,each of which are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Nonwoven filter media are generally composed of randomly oriented fiberswith diameters of about a few dozen micrometers. Although nonwovenfilter media can achieve high filtration efficiency (e.g., removingapproximately 90% of micron-sized particles), they are generally limitedto use in pre-filters and are not used further downstream as highperformance filters. Particularly, the most penetrating particle sizebetween 0.1 and 0.5 μm are not removed by the present nonwoven filtermedia because the size of pores formed with micron-scale fibers isconsiderably larger. To enhance the filtration efficiency of the filtermedia, one generally makes much thicker media for smaller pore sizes.However, thicker filter media can be difficult to use due to increasedpressure drop, and thus higher energy cost. Additionally, fibers andnon-woven fibers are investigated for potential uses in other areas.

SUMMARY OF THE INVENTION

In certain embodiments, provided herein are nanofibers, includingnon-woven nanofiber mats. In some embodiments, the nanofibers comprise acontinuous matrix material along with discrete domains of a secondmaterial. In certain instances, the discrete domains comprise domains ofhigh charge density. In some embodiments, the discrete domains comprisenanoparticles (e.g., charged or chargeable nanoparticles). In certainembodiments, the nanoparticles are metal, metal oxide, or ceramicnanoparticles. In certain instances, wherein the domains and/ornanoparticles are charged or chargeable domains and/or nanoparticles,the nanofibers are useful in filter media. In other instances, thenanoparticles are not charged and may be used in other technologies(e.g., chemistries, such as catalytic chemistries, at the discretedomains of the fibers).

In one aspect, the present invention fulfills a need for filtrationmedia that are capable of both high performance (e.g., removal ofparticle sizes between 0.1 and 0.5 μm) with a low pressure drop, howeverthe invention is not limited in this regard. Described herein arenanofibers and methods for making nanofibers comprising any one or moreof (a) a non-homogeneous charge density; (b) a plurality of regions ofhigh charge density; and/or (c) charged nanoparticles or chargeablenanoparticles.

In one aspect, described herein are nanofibers comprising any one ormore of: (a) a non-homogeneous charge density; (b) a plurality ofregions of high charge density; and/or (c) charged and/or chargeablenanoparticles.

In some embodiments, the standard deviation of charge density of thenanofiber is at least 300% of the net charge density of the nanofiber.In some embodiments, the standard deviation of charge density iscalculated from a plurality of substantially uniformly distributedregions of the nanofiber. In some embodiments, the average thermallystimulated current (TSC) of the nanofiber is at least 200% of the TSC ofthe nanofiber without charged nanoparticles or chargeable nanoparticles.In some embodiments, the charge density of the region of high chargedensity is at least 300% of the net charge density of the nanofiber.

In some embodiments, the regions of high charge density have an averagediameter of about 10 nm. In some embodiments, the regions of high chargedensity are separated by at most 0.05 μm on average. In someembodiments, the regions of high charge density comprise at most 5% ofthe volume of the nanofiber. In some embodiments, the regions of highcharge density comprise at most 5% of the area of the nanofiber.

In some embodiments, the regions of high charge density are distributedsubstantially uniformly on the nanofiber.

In some embodiments, a collection of the nanofibers has a thermallystimulated current (TSC) between about 10⁻¹³ and 10⁻¹² Amp per 5□10⁻⁴ m²of surface area. In some embodiments, a collection of the nanofibers hasa thermally stimulated current (TSC) between about 10⁻¹³ and 10⁻¹² Ampper 5□10⁻⁴ m² of surface area, wherein the collection has a thickness of20 microns and a porosity of 0.5.

In some embodiments, each of the plurality of regions of high chargedensity comprises a nanoparticle. In some embodiments, the metal oxideis selected from the group consisting of MgO, TiO₂, CuO, ZnO and ZrO₂.In some embodiments, the nanoparticles comprise metal oxide. In otherembodiments, the nanoparticle comprises a metal (e.g., a transitionmetal or metalloid, an elemental metal or an alloy, or the like). Insome embodiments, the nanoparticles comprise protein. In someembodiments, the protein is selected from the group consisting of soyprotein and whey protein.

In some embodiments, the nanoparticles are incorporated into an organicsolvent soluble polymer (e.g. a polyacrylonitrile polymer). In someembodiments, the nanoparticles are incorporated into water solublepolymer (e.g. a polyvinyl alcohol polymer). In some embodiments, themass of the nanoparticles is at most 50% of the mass of the nanofiber(or of a fluid stock in a method described herein).

In some embodiments, the nanofiber has a net charge. In someembodiments, the nanofiber is substantially neutral (i.e. does not havea net charge). In some embodiments, the nanofiber has a diameter of atmost 1,000 nm. In some embodiments, the nanofiber has an aspect ratio ofat least 100.

In some embodiments, the invention includes the nanofiber describedherein comprising a coating, wherein the coating comprises chargednanoparticles. In some embodiments, the nanofiber comprises a pluralityof hydrophobic regions. In some embodiments, the nanofiber comprisesprotein, nanoclays, biochar, or any combination thereof.

In one aspect, described herein are filters comprising the nanofibersdescribed herein. In one aspect, described herein are filters suitablefor selectively retaining charged particulates, large particulates andhydrophobic particulates. In one aspect, described herein are filterssuitable for retaining at least 95% of fluid bound particulateschallenged against the filter, wherein the pressure drop across thefilter medium is at most 4 PSI at an air velocity of 80 cm/s. In someembodiments, the particulates have a diameter between about 1 and 30 nm.In some embodiments, the density of the nanofibers comprising the filteris at most 0.5 g/ft². In some embodiments, described herein are filterscomprising the nanofibers described herein. In some embodiments, thenanofibers are disposed on a substrate. In some embodiments, the filteris configured to filter air in a heating ventilation and airconditioning (HVAC) system. In some embodiments, the nanofibers arenon-woven. In some embodiments, the filter is suitable for capturingviruses, microbial organisms (e.g. bacteria), chemicals (e.g.pesticides), or any combination thereof.

In one aspect, described herein is a device comprising the nanofibersdescribed herein, wherein the device is a filter, a membrane, or asensor.

In one aspect, described herein are methods for making a chargednanofiber, the method comprising electrospinning a nanofiber fluid stockcomprising charged nanoparticles or chargeable nanoparticles. In oneaspect, described herein are methods for making a charged nanofiber, themethod comprising depositing charged nanoparticles or chargeablenanoparticles onto a nanofiber.

In some embodiments, the nanofiber comprises protein, nanoclays,biochar, or any combination thereof. In some embodiments, thenanoparticles comprise metal oxide. In some embodiments, the metal oxideis selected from the group consisting of MgO, TiO₂, CuO, ZnO and ZrO₂.In some embodiments, the nanoparticles comprise protein. In someembodiments, the protein is selected from the group consisting of soyprotein and whey protein.

In some embodiments, the nanoparticles are incorporated into an organicsolvent soluble polymer (e.g. a polyacrylonitrile polymer). In someembodiments, the nanoparticles are incorporated into water solublepolymer (e.g. a polyvinyl alcohol polymer). In some embodiments, themass of the nanoparticles is at most 50% of the mass of the nanofiber.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 shows a multi-channel filtration efficiency tester usingpotassium chloride particles.

FIG. 2A shows an SEM image of as-spun fibers with pure PAN fibers.

FIG. 2B shows an SEM image of as-spun fibers with PAN/TiO2 (3.0 vol %)hybrid fibers.

FIG. 2C shows an SEM image of as-spun fibers with PAN/TiO2 (7.5 vol %)hybrid fibers.

FIG. 2D shows an SEM image of as-spun fibers with PAN/TiO2 (13 vol %)hybrid fibers.

FIG. 3 shows diameters of the as-spun fibers (the error bar representsthe standard deviation of the fiber diameter, n=30).

FIG. 4 shows XRD patterns of TiO2 particles and pure PAN fiber andPAN/TiO2 hybrid fibers with different volume fraction of TiO2 particles.

FIG. 5A shows an image of as-spun PAN/TiO2 (7.5) fibers.

FIG. 5B shows an image of Ti information at the same area of the spunfibers of FIG. 5A using EMPA.

FIG. 6 shows thermally simulated current (TSC) spectra of as-spunnanofibers near room temperature.

FIG. 7 shows time dependencies of TSC results of as-spun nanofibers at19° C.: (a) no sample, (b) pure PAN fibers, (c) PAN/TiO2 fibers (3.0 vol%), (d) PAN/TiO2 fibers (7.5 vol %), (e) PAN/TiO2 fibers (13 vol %) (redline—temperature profile, yellow (positive current) and blue line(negative current)—monitored current signals).

FIG. 8 shows pore size distribution of the cellulose substrates coveredwith the varied weight spun fibers.

FIG. 9 shows pressure drop difference (PSI) with bare filter atdifferent volume air flow (L/min) measured by a capillary flowporometry.

FIG. 10A shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate before filtration testing for 0.1g/ft² PAN fibers (scale bar is 3 μm).

FIG. 10B shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate after filtration testing for 0.1g/ft2 PAN fibers (scale bar is 3 μm).

FIG. 10C shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate before filtration testing for 0.5g/ft2 PAN fibers (scale bar is 3 μm).

FIG. 10D shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate after filtration testing for 0.5g/ft2 PAN fibers (scale bar is 3 μm).

FIG. 10E shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate before filtration testing for 0.1g/ft2 PAN/TiO2 hybrid fibers (scalebar is 3 μm).

FIG. 10F shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate after filtration testing for 0.1g/ft2 PAN/TiO2 hybrid fibers (scale bar is 3 μm).

FIG. 10G shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate before filtration testing for 0.5g/ft2 PAN/TiO2 hybrid fibers (scale bar is 3 μm).

FIG. 10H shows an SEM image of PAN nanofibers and PAN/TiO2 hybridnanofibers on the cellulose substrate after filtration testing for 0.5g/ft2 PAN/TiO2 hybrid fibers (scale bar is 3 μm).

FIG. 11 shows typical results of filtration efficiency tests withPAN/TiO2 (0.25 g/ft²), showing the particle counts for upstream anddownstream measurement.

FIG. 12 shows filtration efficiency (%) of the various substratescovered with different fiber mass as a function of particle size.

FIG. 13 shows overall filtration efficiency (particle size 0.1-1.0 □m)according to different fiber masses on the cellulose substrates andtheir pore diameters.

FIG. 14 shows a method for producing nanofibers comprising nanoparticles(e.g., depositing metal oxide nanoparticles directly on a filtrationmedia).

FIG. 15 shows a plot of zeta potential in mV versus pH for PAN fiberswith and without TiO₂ nanoparticles.

FIG. 16 shows a plot of pressure drop in PSI versus air velocity in cm/sfor filtration media comprising PAN at various densities, with andwithout TiO₂ nanoparticles.

FIG. 17A illustrates a TEM image of certain nanofibers prepared byelectrospinning a fluid stock comprising polymer and nanoparticleswithout a gas-assisted process described herein, and aggregation ofnanoparticles in resultant fibers.

FIG. 17B illustrates a TEM image of certain nanofibers prepared byelectrospinning a fluid stock comprising polymer and nanoparticleswithout a gas-assisted process described herein, and aggregation ofnanoparticles in resultant fibers.

FIG. 17C illustrates a TEM image of certain nanofibers prepared byelectrospinning a fluid stock comprising polymer and nanoparticleswithout a gas-assisted process described herein, and aggregation ofnanoparticles in resultant fibers.

FIG. 17D illustrates a TEM image of certain nanofibers prepared byelectrospinning a fluid stock comprising polymer and nanoparticleswithout a gas-assisted process described herein, and aggregation ofnanoparticles in resultant fibers.

FIG. 18 illustrates a TEM image of nanofibers prepared using gasassisted electrospinning, and the non-aggregation of nanoparticleswithin a nanofiber matrix resulting therefrom.

FIG. 19 illustrates an exemplary co-axial electrospinning apparatus forgas assisted electrospinning described herein.

DETAILED DESCRIPTION OF THE INVENTION

Described herein are nanofibers and methods for making nanofiberscomprising any one or more of (a) a non-homogeneous charge density; (b)a plurality of regions (e.g., non-aggregated regions/domains) of highcharge density; and/or (c) charged nanoparticles or chargeablenanoparticles. In specific embodiments, provided herein are nanofibersand methods of making nanofibers, the nanofibers comprising a pluralityof nanoparticles (e.g., non-aggregated nanoparticles), the nanoparticlesbeing charged. In some embodiments, provided herein are nanofibers andmethods for making nanofibers comprising a plurality of nanoparticles(e.g., metal containing nanoparticles, such as metal, metal oxide, orceramic nanoparticles).

In one aspect, the nanofibers provided herein fulfill a need forfiltration media that are capable of both high performance (e.g.,removal of particle sizes between 0.1 and 0.5 μm) with a low pressuredrop, however the invention is not limited in this regard. Highefficient particulate air (HEPA) filters have a high filtrationefficiency (99.97%) for particles greater than or equal to 0.3 μm indiameter. HEPA performance can be achieved with nylon 6 nanofibrousfilter media (made of 80-200 nm diameter fiber) having a coveragedensity of 1.0 g/ft², however the air flow rate is significantlydecreased and there is a considerable increase of pressure drop causingthe short operating life of the filter media. In some embodiments, thenanofibers and methods for making nanofibers described herein achieveHEPA performance (or better) with a low pressure drop. In other aspects,nanofibers provided herein provide nanostructured materials meeting arange of needs, particularly wherein non-aggregated domains (e.g.,nanoparticles) or non-aggregated charged domains (e.g., nanoparticles)are useful.

Electrospinning

In certain embodiments, provided herein are processes for preparingnanofibers described herein. In some embodiments, the process compriseselectrospinning a fluid stock. In specific embodiments, the fluid stockcomprises a polymer. In various embodiments, the fluid stock comprises amelted polymer. In other embodiments, the fluid stock comprises apolymer in a solvent. In specific embodiments, the solvent is an organicsolvent (e.g., a polar solvent, such as dimethylformamide (DMF)). Insome specific embodiments, the solvent is aqueous or water. In specificembodiments, the solvent is water and the polymer is water soluble(e.g., dissolving in water or at least swelling in the water). In morespecific embodiments, the solvent is water, the polymer is watersoluble, and the nanoparticles are metal nanoparticles. In otherspecific embodiments, the solvent is water, the polymer is watersoluble, and the nanoparticles are metal oxide nanoparticles.

In some embodiments, the nanofibers are produced by electrospinning.Electrospinning involves applying an electrical potential between ametal plate and a needle, through which a fluid stock (e.g., comprisinga polymer solution) is fed. The potential difference draws the fluidstock into a jet in the direction of the plate. The jet then undergoes awhipping motion due to electrostatic repulsion between its own surfaces,drawing it out to submicron scale diameters. As the fluid stock jettravels toward the plate, the solvent evaporates, producing a mat ofnon-woven polymer nanofibers. In some embodiments, these nanofibrousmats are characterized by a large surface area-to-volume ratio, smallpore size, and high porosity compared to non-woven fabrics. As such, insome embodiments they are used for filtration of particulate matter fromair, and when spun onto a substrate made of relatively thick fibers,they increase filtration dramatically (e.g., in capturing and holdingparticles of sub-micron size) as described herein.

The nanofibers and methods for making nanofibers described hereincomprise charged (or chargeable) nanoparticles in some embodiments. Insome embodiments, the nanofibers and/or charged nanoparticles aredeposited directly onto a supporting filter medium (e.g., FIG. 14 ).FIG. 14 illustrates spun nanofiber (e.g., directly deposited on a filtersupport). In some instances, the spun nanofiber comprises a polymermatrix, and nanoparticle (e.g., charged nanoparticle) embedded in thepolymer matrix. FIG. 14 illustrates an exemplary system or schematic ofa process described herein, particularly a system or process forpreparing a nanofiber (e.g., by a coaxial gas assisted electrospinningprocess). In some instances, a fluid stock (e.g., comprisingpolymer—such as solvent soluble polymer (PAN, N6, PLA, or the like) andnanoparticles—such as metal oxide nanoparticles (MgO, TiO₂, Al₂O₃, CuO,ZnO, ZrO₂, or the like other metal component described herein)) isprepared by combining nanoparticles with polymer and optional solvent(e.g., for dissolving the polymer). In some embodiments, the fluid stockis provided to an electrospinning apparatus having a needle apparatus(e.g., a coaxial needle apparatus as illustrated in FIG. 19 ). Incertain instances, the electrospinning is gas assisted (e.g., the fluidis provided through the central needle 1903 and the gas is providedthrough the outer needle 1902—additional needles aligned along a commonaxis are optional for providing additional fluid stocks, gas, or otherfluids—such as for nanofiber coats). The fluid stocks may be provided toan electrospinning apparatus (e.g., an electrospinning needle apparatuswith voltage supplied thereto—e.g., voltage sufficient to overcome thesurface tension of a liquid polymer or polymer solution to produce ajet) by any device, e.g., by a syringe or a pump. A gas may be providedto an electrospinning needle apparatus from any source (e.g., air pump).

In one aspect, described herein is a method for making a chargednanofiber, the method comprising electrospinning a nanoparticle fluidstock comprising charged nanoparticles or chargeable nanoparticles. Inanother aspect, described herein is a method for making a chargednanofiber, the method comprising depositing charged nanoparticles orchargeable nanoparticles onto a nanofiber. In specific embodiments, themethod comprises electrospinning a fluid stock, the fluid stockcomprising a polymer and a plurality of nanoparticles (e.g., chargeablenanoparticles). In specific instances, electrospinning of the nanofibers(i.e., applying an electric charge to the fluid stock to form a jet)causes the nanoparticles to become charged, resulting in an electrospunnanofiber comprising charged nanoparticles embedded in a polymer matrix.In certain instances, gas assisted electrospinning of the nanofiber isutilized, providing an electrospun nanofiber comprising non-aggregatedcharged nanofibers embedded in a polymer matrix.

Any suitable method for electrospinning is used. For example, elevatedtemperature electrospinning is described in U.S. Pat. No. 7,326,043filed on Oct. 18, 2004; U.S. patent application Ser. No. 13/036,441filed on Feb. 28, 2011; and U.S. Pat. No. 7,901,610 filed on Jan. 10,2008, which are incorporated herein for such disclosure. In someembodiments, the electro-spinning is gas-assisted as described in PCTPatent Application PCT/US11/24894 filed on Feb. 15, 2011, which isincorporated herein by reference for such disclosure. Briefly,gas-assisted electrospinning comprises expelling a stream of gas at highvelocity along with the fluid stock (e.g., as a stream inside thefluidstock or surrounding the fluid stock), which increases thethrough-put of an electrospinning process. In some instances, gasassisted electrospinning accelerates and elongates the jet of fluidstock emanating from the electrospinner. In some instances, gas assistedelectrospinning disperses nanoparticles in nanocomposite nanofibers. Forexample, in some instances, gas assisted electrospinning (e.g., coaxialelectrospinning of a gas—along a substantially common axis—with a fluidstock comprising nanoparticles) facilitates dispersion ornon-aggregation of the nanoparticles in the electrospun jet and theresulting as-spun nanofiber (and subsequent nanofibers producedtherefrom). In some embodiments, incorporating a gas stream inside afluid stock produces hollow nanofibers. In some embodiments, the fluidstock is electrospun using any suitable technique.

In certain instances, gas assistance of the electrospinning of ananoparticle containing fluid stock increases fluid throughput andreduces or prevents nanoparticle aggregation in the needle apparatus,thereby reducing or preventing nanoparticle aggregation in the as-spunfiber. In specific embodiments, the nanofibers comprises less than 50%of nanoparticles that are aggregated. In specific embodiments, thenanofibers comprises less than 40% of nanoparticles that are aggregated.In specific embodiments, the nanofibers comprises less than 25% ofnanoparticles that are aggregated. In specific embodiments, thenanofibers comprises less than 10% of nanoparticles that are aggregated.In specific embodiments, the nanofibers comprises less than 5% ofnanoparticles that are aggregated.

FIGS. 17A-D illustrate certain nanofibers prepared by electrospinning afluid stock comprising polymer and nanoparticles without a gas-assistedprocess described herein. FIG. 18 illustrate a nanofiber 1801 havingnon-aggregation of nanoparticles 1801 within the matrix/backbonematerial 1802, whereas FIGS. 17 A-D illustrate aggregation ofnanoparticles within the matrix material.

FIG. 19 illustrates co-axial electrospinning apparatus 1900. The coaxialneedle apparatus comprises an inner needle 1901 and an outer needle1902, both of which needles are coaxially aligned around a similar axis1903 (e.g., aligned with 5 degrees, 3 degrees, 1 degree, or the like).In some embodiments, further coaxial needles may be optionally placedaround, inside, or between the needles 1901 and 1902, which are alignedaround the axis 1903. In some instances, the termination of the needlesis optionally offset 1904.

In some embodiments, the method comprises co-axially electrospinning afirst fluid stock with a second fluid stock to produce a nanofiberwherein at least one of the fluid stocks comprises charged or chargeablenanoparticles. Co-axial electrospinning is described in PCT PatentApplication PCT/US11/24894 filed on Feb. 15, 2011, which is incorporatedherein by reference for such disclosure. In some embodiments, the firstfluid stock comprises nanoparticles, and the first fluid stock at leastpartially coats the second fluid stock.

Fluid Stocks

In some aspects the nanofibers and methods described herein compriseelectrospinning a fluid stock. In some embodiments, the fluid stocks aresolvent-based (e.g., comprise an organic solvent such as DMF) or aqueous(i.e., water-based). In some embodiments, fluid stocks suitable forproducing metal, ceramic, metal alloy, or any combination thereof (i.e.,hybrid nanofibers) comprise a water soluble polymer and precursormolecules distributed substantially uniformly on the polymer asdescribed in U.S. Provisional Patent Application No. 61/528,895 filed onAug. 30, 2011, International Patent Application PCT/US12/53097, filed onAug. 30, 2012, and U.S. Provisional Patent Application No. 61/701,903,each of which are incorporated herein for such disclosure. Theprecursors are metal precursor, ceramic precursor, carbon precursor,nanoparticles, or any combination thereof in various embodiments. Asused herein, a “carbon precursor” is a polymer (e.g.,polyacrylonitrile), wherein thermal treatment of the electrospun fluidstock is capable of converting the carbon precursor into a carbonnanofiber. Other suitable fluid stocks include nanoclays as described inU.S. Pat. No. 7,083,854 filed on May 10, 2005 or inorganic fluid stocksas described in U.S. patent application Ser. No. 11/694,435 filed onMar. 30, 2007 or fluid stocks comprising protein as described in PCTPatent Application No. PCT/US10/35220 filed on May 18, 2010.

In some embodiments, the metal precursor comprises metal selected fromthe group consisting of: scandium (Sc), titanium (Ti), vanadium (V),chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni),copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb),molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh),palladium (Pd), silver (Ag), cadmium (Cd), hafnium (Hf), tantalum (Ta),tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt),gold (Au), mercury (Hg), rutherfordium (Rf), dubnium (Db), seaborgium(Sg), bohrium (Bh), and hasium (Hs). Suitable alkali metals include:lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs) andfrancium (Fr). Suitable alkaline earth metals include: beryllium (Be),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium(Ra). Suitable post-transition metals include: aluminum (Al), gallium(Ga), indium (In), tin (Sn), thallium (Tl), lead (Pb), and bismuth (Bi).Suitable lanthanides include the elements with atomic No. 57 to 71 onthe periodic table. Suitable actinides include the elements with atomicNo. 89 to 103 on the periodic table. In some embodiments, suitablemetals also include metalloids, such as germanium (Ge), antimony (Sb),polonium (Po), or silicon (Si). The nanoparticle may additionally begermanium (Ge), antimony (Sb) and polonium (Po), silicon (Si), or carbon(C). In further or alternative embodiments, the metal precursorcomprises a transition metal, or metalloid. In further or alternativeembodiments, the metal precursor is a metal-ligand complex comprisingone or more ligand selected from the group consisting of: a carboxylate,a nitrate, a halide, a diketone, an alkoxide, and combinations thereof.In further or alternative embodiments, the reagent precursor, theprecursor of the fluid stock, or both comprise one or more metalacetate, metal nitrate, metal chloride, metal methoxide, or acombination thereof. In certain embodiments, a metal precursor loaded inthe fluid stock and thermally treated after preparation is suitable forpreparing a nanofiber with a continuous metal matrix (e.g., if thermallytreated—e.g., 400 to 1200 C—under inert or reducing conditions), ananofiber with a continuous metal oxide matrix, (e.g., if thermallytreated under oxidizing—e.g., air—conditions), a nanofiber with a metalcarbide matrix (e.g., if thermally treated at high temperatures—e.g., atleast 1200 C), or the like.

Nanofibers

Described herein are nanofibers and methods for making nanofiberscomprising any one or more of (a) a non-homogeneous charge density; (b)a plurality of regions of high charge density; and/or (c) chargednanoparticles or chargeable nanoparticles.

In various embodiments, the nanofibers described herein comprise anysuitable material. In various embodiments, the methods described hereincan be used to make nanofibers comprising any suitable material.Exemplary materials include protein, polymer, nanoclays, biochar or anycombination thereof. Methods for producing metal, ceramic, metal alloyand hybrid nanofibers including methods for calcinating nanofibers aredescribed in U.S. Provisional Patent Application 61/528,895 filed onAug. 30, 2011. In addition, virtually any material is allowed forproducing charged nanofibers using the methods of the disclosure. Insome embodiments, the material is convertible from suitable precursorsin the fluid stock. In some embodiments, the nanofiber is a calciumphosphate (Ca—P) nanofiber. In some embodiments, the methods of thepresent disclosure may produce charged Ca—P nanofibers.

In some embodiments, the methods of the present disclosure are combinedwith other methods to produce yet more embodiments of the presentdisclosure. For example, the nanofiber is surface-modified. For example,enzymes are immobilized on the nanofiber surface to create a biologicalcatalyst. In another example, doping processes from the semiconductorindustry are employed to intentionally introduce impurities into anextremely pure semiconductor nanofiber for the purpose of modulating itselectrical properties.

The nanofiber has any suitable diameter. In some embodiments, thediameter is about 20 nm, about 50 nm, about 100 nm, about 200 nm, about300 nm, about 500 nm, or about 1,000 nm. In some embodiments, thediameter is at most 20 nm, at most 50 nm, at most 100 nm, at most 200nm, at most 300 nm, at most 500 nm, at most 1,000 nm, at most 1.5microns, at most 2 microns, or the like. The nanofiber has any suitableaspect ratio (ratio of length to diameter). In some embodiments, theaspect ratio is at least 20, at least 50, at least 100, at least 300, atleast 500, at least 1000, at least 10,000, or the like. In someembodiments, nanofibers provided herein have a (e.g., average) length ofat least 10 μm, at least 20 μm, at least 100 μm, at least 500 μm, atleast 1,000 μm, at least 5,000 μm, at least 10,000 μm, or the like.

In some embodiments, the nanofiber is coated. In some embodiments, thecoating comprises charged nanoparticles. The nanofiber is hydrophilic insome embodiments. In some embodiments, the nanofiber is hydrophobic. Insome embodiments, the nanofiber comprises a plurality of hydrophobicregions.

Nanofiber Charge Characteristics

In some embodiments, the nanofibers described herein have a net charge(i.e., a non-zero charge). The net charge is positive in someembodiments. The net charge is negative in some embodiments. Themagnitude of the charge (positive or negative) can be any suitablevalue. In some embodiments, the nanofiber is substantially neutral (i.e.does not have a net charge).

In some embodiments, the nanofibers have a non-homogeneous chargedensity. Non-homogenous charge density is possible even when thenanofibers are (overall) neutrally charged (i.e., the nanofibers haveregions of positive charge and negative charge). Nanofibers with anon-homogenous charge density and methods for producing nanofibers witha non-homogenous charge density are useful in filtration applicationsfor example. In some instances, e.g., in filtration, negatively chargedparticles (e.g., contaminants or pollutants) are attracted to regions ofhighly positive charge density, allowing high filtration efficiencieswith less nanofiber material and a low pressure drop as exemplified anddescribed herein.

In one aspect, the nanofibers described herein have a charge densitywith a high standard deviation. The standard deviation is calculatedfrom a plurality of substantially uniformly distributed regions of thenanofiber. The regions are surface regions, measured in terms of area insome embodiments. In other embodiments, the regions are volumetricregions. The regions are substantially evenly distributed if thedistances from one region to its closest neighboring region vary by atmost 10%, at most 20%, at most 50% in various embodiments. The standarddeviation is high in some embodiments, indicating a highlynon-homogenous charge distribution. In some embodiments, the standarddeviation of the charge density of the nanofiber is about 50%, about100%, about 200%, about 300%, about 400%, about 500%, about 700%, about1,000%, and the like of the net charge density of the nanofiber. In someembodiments, the standard deviation of the charge density of thenanofiber is at least 50%, at least 100%, at least 200%, at least 300%,at least 400%, at least 500%, at least 700%, at least 1,000%, and thelike of the net charge density of the nanofiber.

In some embodiments, nanofibers provided herein comprise domains of highcharge density (e.g., charged nanoparticles) embedded within acontinuous matrix (e.g., a polymer matrix, a carbonized polymer matrix,a metal (e.g., prepared from calcined metal precursor), a ceramic (e.g.,prepared from calcined and oxidized metal precursor), or the like). Inspecific embodiments, the domains of high charge density arenon-aggregated. In specific embodiments, the nanofibers comprises lessthan 50% of highly charged domains (e.g., charged nanoparticles) thatare aggregated. In specific embodiments, the nanofibers comprises lessthan 40% of highly charged domains that are aggregated. In specificembodiments, the nanofibers comprises less than 25% of highly chargeddomains that are aggregated. In specific embodiments, the nanofiberscomprises less than 10% of highly charged domains that are aggregated.In specific embodiments, the nanofibers comprises less than 5% of highlycharged domains that are aggregated.

In some embodiments, highly charged domains provided herein have acharge at least 2 times, at least 3 times, at least 5 times, at least 10times, at least 20 times, or more of the charge of the low-charged(e.g., non- or lower-charged) matrix. In certain embodiments, highlycharged domains provided herein have a charge at least 2 times, at least3 times, at least 5 times, at least 10 times, at least 20 times, or moreof the charge of the overall average charge (e.g., per unit size of theaverage highly charged domain) of the nanofiber.

In some embodiments, a nanocomposite nanofiber provide herein comprisesdiscrete domains within the nanocomposite nanofiber. In specificembodiments, the discrete domains comprise a metal, metal oxide, orceramic material (e.g., a charged or chargeable material). In certainembodiments, the discrete domains are non-aggregated. In someembodiments, the non-aggregated domains are dispersed, e.g., in asubstantially uniform manner, along the length of the nanofiber. Incertain embodiments, the nanocomposite nanofibers provided herein do notcomprise a concentration of domains in one segment (e.g., a 500 nm, 1micron, 1.5 micron, 2 micron) segment that is over 10 times (e.g., 20times, 30 times, 50 times, or the like) as concentrated as animmediately adjacent segment. In some embodiments, the segment size forsuch measurements is a defined length (e.g., 500 nm, 1 micron, 1.5micron, 2 micron). In other embodiments, the segment size is a functionof the average domain (e.g., particle) size (e.g., the segment 5 times,10 times, 20 times, 100 times the average domain size). In someembodiments, the domains have a (average) size 1 nm to 1000 nm, 1 nm to500 nm, 1 nm to 200 nm, 1 nm to 100 nm, 20 nm to 30 nm, 1 nm to 20 nm,30 nm to 90 nm, 40 nm to 70 nm, 15 nm to 40 nm, or the like.

In some embodiments, the nanofibers comprise non-aggregated discretedomains. In certain embodiments, the nanofibers do not comprise aconcentration of domains 20 times higher along a 500 nm long segmentalong the length of the nanofiber than an adjacent 500 nm length of thenanofiber. In some embodiments, the discrete domains comprise at least90% by weight metal having an oxidation state of zero. In specificembodiments, the discrete domains comprise at least 95% by weight metalhaving an oxidation state of zero. In other embodiments, the discretedomains comprise at least 90% by weight metal having an oxidation stateof greater than zero (e.g., a metal oxide). In specific embodiments, thediscrete domains comprise at least 95% by weight metal having anoxidation state of greater than zero (e.g., a metal oxide).

Thermally stimulated current (TSC) is one suitable method for measuringthe electrical properties of the nanofiber (e.g., residual current andthus charge density). Two exemplary TSC methods are described in Example6. In some embodiments, the nanofibers comprise charged or chargeablenanoparticles. In some embodiments, the average thermally stimulatedcurrent (TSC) of the nanofiber is about 150%, about 200%, about 300%,about 400%, about 500%, about 1,000%, and the like of the TSC of thenanofiber without charged nanoparticles or chargeable nanoparticles. Insome embodiments, the nanofibers comprise charged or chargeablenanoparticles. In some embodiments, the average thermally stimulatedcurrent (TSC) of the nanofiber is at least 150%, at least 200%, at least300%, at least 400%, at least 500%, at least 1,000%, and the like of theTSC of the nanofiber without charged nanoparticles or chargeablenanoparticles.

In some embodiments, TSC is measured on a collection of the nanofibersdescribed herein. In some embodiments, the collection of nanofibers hasa thickness of 20 microns and a porosity of 0.5. The TSC has anysuitable value. In some embodiments, the TSC of a disk (one inch ofdiameter and 20 □m in thickness) of a collection of nanofibers has a TSCof about 5□10⁻¹¹, about 1□10⁻¹², about 5□10⁻¹², about 1□10⁻¹³, about5□10⁻¹³, and the leAmps per bulk surface area (5□10⁻⁴ m²). In someembodiments, the TSC of a one inch disk region of a collection ofnanofibers has a TSC of at least 5□10⁻¹¹, at least 1□10⁻¹², at least5□10⁻¹², at least 1□10⁻¹³, at least 5□10⁻¹³, and the like Amps per bulksurface area (5□10⁻⁴ m²). In some embodiments, the TSC of a one inchdisk region of a collection of nanofibers has a TSC of between about1□10⁻¹² and 1□10⁻¹³ Amps per bulk surface area (5□10⁻⁴ m²).

Discrete Domains and Regions of High Charge Density

In one aspect, described herein are charged nanofibers and methods formaking charged nanofibers having a plurality of regions of high chargedensity. The regions of high charge density are comprised of anysuitable material and are made or formed in any suitable manner. In someembodiments, each of the plurality of regions of high charge densitycomprises a nanoparticle. In other embodiments, the regions are formedby a chemical etching process, by laser modification of the surface ofthe nanofiber, or any other suitable method. In some embodiments, theregions of high charge density are volumetric regions (and the chargedensity has units of charge per volume). In some embodiments, theregions of high charge density are surface regions (and the chargedensity has units of charge per area).

In some embodiments, the nanofibers comprises regions of high chargedensity and the regions of high charge density have any suitable chargedensity. In some embodiments, the charge density of the region of highcharge density is about 200%, about 300%, about 500%, about 1,000%,about 5,000%, about 10,000%, and the like of the net charge density ofthe nanofiber. In some embodiments, the charge density of the region ofhigh charge density is at least 200%, at least 300%, at least 500%, atleast 1,000%, at least 5,000%, at least 10,000%, and the like of the netcharge density of the nanofiber.

In various embodiments, nanofibers described herein have a continuousmaterial and discrete domains (“regions”) of a second material. In someembodiments, the second material constitutes or comprises a region ofhigh charge density. In various embodiments, these regions or domains(e.g., the regions of high charge density) have any suitable diameter.The regions (e.g., of charge density) have any suitable shape. In someembodiments, the diameter is measured by microscopy. In someembodiments, an average diameter is calculated from a plurality ofregions (e.g., of high charge density). In some embodiments, the regions(e.g., of high charge density) have an average diameter of about 3 nm,about 5 nm, about 10 nm, about 20 nm, about 50 nm, about 100 nm, about200 nm, about 400 nm, about 1,000 nm, and the like. In some embodiments,the regions (e.g., of high charge density) have an average diameter ofat least 3 nm, at least 5 nm, at least 10 nm, at least 20 nm, at least50 nm, at least 100 nm, at least 200 nm, at least 400 nm, at least 1,000nm, and the like. In some embodiments, the regions (e.g., of high chargedensity) have an average diameter of at most 3 nm, at most 5 nm, at most10 nm, at most 20 nm, at most 50 nm, at most 100 nm, at most 200 nm, atmost 400 nm, at most 1,000 nm, and the like.

In certain embodiments, the regions (e.g., of high charge density) areseparated from each other by any suitable distance. The separationdistance may be determined in any suitable manner, such as bymicroscopy. In some embodiments, the distances from each region (e.g.,of high charge density) to its nearest (e.g., high charge) region areaveraged. In some embodiments, the regions (e.g., of high chargedensity) are separated by about 0.05 μm, about 0.1 μm, about 0.5 μm,about 1 μm, about 5 μm, about 10 μm, about 50 μm, about 100 μm, and thelike on average. In some embodiments, the regions (e.g., of high chargedensity) are separated by at least 0.05 μm, at least 0.1 μm, at least0.5 μm, at least 1 μm, at least 5 μm, at least 10 μm, at least 50 μm, atleast 100 μm, and the like on average.

In some embodiments, efficient filtration is achieved using a relativelysmall or few regions of high charge density. The regions of high chargedensity comprise any suitable percentage of the volume of the nanofiber.In some embodiments, the total volume of the regions of high chargedensity comprise about 0.1%, about 0.5%, about 1%, about 5%, about 10%,about 25%, about 50%, and the like of the volume of the nanofiber. Insome embodiments, the total volume of the regions of high charge densitycomprise at least 0.1%, at least 0.5%, at least 1%, at least 5%, atleast 10%, at least 25%, at least 50%, and the like of the volume of thenanofiber. In some embodiments, the total volume of the regions of highcharge density comprise at most 0.1%, at most 0.5%, at most 1%, at most5%, at most 10%, at most 25%, at most 50%, and the like of the volume ofthe nanofiber.

In some embodiments, efficient filtration is achieved using a relativelysmall or few regions of high charge density. The regions of high chargedensity comprise any suitable percentage of the surface area of thenanofiber. In some embodiments, the total surface area of the regions ofhigh charge density comprise about 0.1%, about 0.5%, about 1%, about 5%,about 10%, about 25%, about 50%, and the like of the surface area of thenanofiber. In some embodiments, the total surface area of the regions ofhigh charge density comprise at least 0.1%, at least 0.5%, at least 1%,at least 5%, at least 10%, at least 25%, at least 50%, and the like ofthe surface area of the nanofiber. In some embodiments, the totalsurface area of the regions of high charge density comprise at most0.1%, at most 0.5%, at most 1%, at most 5%, at most 10%, at most 25%, atmost 50%, and the like of the surface area of the nanofiber.

In some instances, the regions (e.g., of high charge density) aredistributed substantially uniformly on the nanofiber. Here, in someembodiments, the regions (e.g., of high charge density) aresubstantially uniformly distributed if the standard deviation of thedistances between a plurality of regions (e.g., of high charge density)from its nearest region of high charge density is about 20%, about 50%,about 100%, and the like of the average of the distances between regions(e.g., of high charge density). In some embodiments, the regions (e.g.,of high charge density) are substantially uniformly distributed if thestandard deviation of the distances between a plurality of regions(e.g., of high charge density) from its nearest region (e.g., of highcharge density) is at most 20%, at most 50%, at most 100%, and the likeof the average of the distances between regions (e.g., of high chargedensity).

Nanoparticles

In one aspect, described herein are charged nanofibers and methods formaking charged nanofibers comprising charged nanoparticles or chargeablenanoparticles. For example, TiO₂ nanoparticles have been widely used invarious applications such as photocatalysts, pigments and cosmeticsadditives, often in media of intermediate polarity. However, the effectof incorporated nanoparticles (such as TiO₂) in nanofibers (e.g.,covering a filter substrate on air filtration efficiency) has beendescribed previously. As described herein, high air filtrationefficiency with a low pressure drop is achieved by covering thesubstrate with polymeric nanofibers containing metal oxidenanoparticles. In some instances, charged nanoparticleselectrostatically interact with dust particles (e.g., increasingaffinity between the nanofiber and the pollutant/dust particle, and insome instances—modulating where on the nanofiber the affinity occurs).

In one embodiment, filtration efficiency of filter media covered withpure PAN nanofibers and PAN/TiO₂ hybrid nanofibers was evaluated, takinginto account properties affecting filtration efficiency, including thestructural properties of filter media (pore diameter, pore sizedistributions, pressure drop) and the electric charge property of theas-spun fibers. As shown herein, despite less fiber coverage density onthe cellulose substrate, the filtration efficiency of the PAN filtermedia with TiO₂ nanoparticles added is much greater than that of thosemade with pure PAN fibers even under less pressure drop. Without beingheld to any particular theory, such enhancement is explained by theadded electric charge interaction between TiO2 nanoparticles innanofibrous filter media and the simulated dust particles, supported bythe thermally stimulated current (TSC) results.

The nanoparticles are made of any suitable material. In someembodiments, the nanoparticles comprise metal oxide. In someembodiments, the metal oxide is selected from the group consisting ofMgO, TiO₂, CuO, ZnO and ZrO₂. In some embodiments, the nanoparticlescomprise protein. In some embodiments, the protein is selected from thegroup consisting of soy protein and whey protein. In certainembodiments, the nanoparticles comprise a metal. In further oralternative embodiments, the nanoparticles comprise a ceramic.

Provided in various embodiments herein are nanofibers comprising puremetal nanoparticles and nanofibers comprising metal nanoparticles. Invarious embodiments, the pure metal nanoparticles have any suitablepercent composition of metal. In some embodiments, the metalnanoparticles comprise about 99.99%, about 99.95%, about 99.9%, about99%, about 98%, about 97%, about 96%, about 95%, about 90%, about 80%,and the like of metal by mass. In some embodiments, the metalnanoparticles comprise at least about 99.99%, at least about 99.95%, atleast about 99.9%, at least about 99%, at least about 98%, at leastabout 97%, at least about 96%, at least about 95%, at least about 90%,at least about 80%, and the like of metal by mass.

In various embodiments, the metal of a metal, metal oxide, or ceramicprovided herein is any suitable metal: transition metal, alkali metal,alkaline earth metal, post-transition metal, lanthanide, or actinide.Suitable transition metals include: scandium (Sc), titanium (Ti),vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co),nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr),niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium(Rh), palladium (Pd), silver (Ag), cadmium (Cd), hafnium (Hf), tantalum(Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum(Pt), gold (Au), mercury (Hg), rutherfordium (Rf), dubnium (Db),seaborgium (Sg), bohrium (Bh), and hasium (Hs). Suitable alkali metalsinclude: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium(Cs) and francium (Fr). Suitable alkaline earth metals include:beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium(Ba), and radium (Ra). Suitable post-transition metals include: aluminum(Al), gallium (Ga), indium (In), tin (Sn), thallium (Tl), lead (Pb), andbismuth (Bi). Suitable lanthanides include the elements with atomic No.57 to 71 on the periodic table. Suitable actinides include the elementswith atomic No. 89 to 103 on the periodic table. In some embodiments,suitable metals also include metalloids, such as germanium (Ge),antimony (Sb), polonium (Po), or silicon (Si). The nanoparticle mayadditionally be germanium (Ge), antimony (Sb) and polonium (Po), silicon(Si), or carbon (C). In some embodiments, the metal is not a metalloid.In specific embodiments, the metal component is not a siliconnanoparticle. In additional or other specific embodiments, the metalcomponent is not a lithium containing nanoparticle.

Provided in various embodiments herein are nanofibers comprising pureceramic nanoparticles and nanofibers comprising ceramic nanoparticles.In some embodiments, the ceramic nanoparticles comprise about 99.99%,about 99.95%, about 99.9%, about 99%, about 98%, about 97%, about 96%,about 95%, about 90%, about 80%, and the like of ceramic by mass. Insome embodiments, the ceramic nanoparticles comprise at least about99.99%, at least about 99.95%, at least about 99.9%, at least about 99%,at least about 98%, at least about 97%, at least about 96%, at leastabout 95%, at least about 90%, at least about 80%, and the like ofceramic by mass.

In some embodiments, the ceramic is a metal oxide. Exemplary ceramicsinclude but are not limited to Al₂O₃, ZrO₂, Fe₂O₃, CuO, NiO, ZnO, CdO,SiO₂, TiO₂, V₂O₅, VO₂, Fe₃O₄, SnO, SnO₂, CoO, CoO₂, Co₃O₄, HfO₂, BaTiO₃,SrTiO₃, and BaSrTiO₃.

Provided in various embodiments herein are nanofibers comprising puremetal alloy nanoparticles and nanofibers comprising metal alloynanoparticles. The metal alloy is allowed to be any metal alloyincluding: transition metal, alkali metal, alkaline earth metal,post-transition metal, lanthanide, or actinide, additionally, germanium(Ge), antimony (Sb) and polonium (Po), and silicon (Si). Exemplary metalalloys include, but are not limited to CdSe, CdTe, PbSe, PbTe, FeNi(perm alloy), Fe—Pt intermetallic compound, Pt—Pb, Pt—Pd, Pt—Bi, Pd—Cu,and Pd—Hf.

In some embodiments, the nanoparticles are incorporated into the fluidstock from which the nanofiber is electrospun. In some embodiments, thenanoparticles are incorporated into an organic solvent soluble polymer(e.g. a polyacrylonitrile polymer). In some embodiments, thenanoparticles are incorporated into water soluble polymer (e.g. apolyvinyl alcohol polymer).

The nanoparticles comprise any suitable fraction of the fluid stock orthe nanofibers. In some embodiments the mass of the nanoparticles isabout 0.1%, about 1%, about 5%, about 10%, about 25%, about 50%, and thelike of the mass of the nanofiber. In some embodiments the mass of thenanoparticles is at most 0.1%, at most 1%, at most 5%, at most 10%, atmost 25%, at most 50%, and the like of the mass of the nanofiber.

Nanofiber Production

The fluid stock has any suitable composition.

In some embodiments, a polymer in a process or nanofiber describedherein is an organic polymer. In some embodiments, polymers used in thecompositions and processes described herein are hydrophilic polymers,including water-soluble and water swellable polymers. In someembodiments, the polymer (e.g., water soluble polymer) used in a fluidstock (e.g., aqueous) or fiber described herein by way of non-limitingexample polyvinyl alcohol (“PVA”), polyvinyl acetate (“PVAc”),polyethylene oxide (“PEO”), polyvinyl ether, polyvinyl pyrrolidone,polyglycolic acid, hydroxyethylcellulose (“HEC”), ethylcellulose,cellulose ethers, polyacrylic acid, polyisocyanate, polyvinylidenedifluoride (“PVDF”), polyacrylate (e.g., polyalkacrylate, polyacrylicacid, polyalkylalkacrylate, or the like), and the like. In someembodiments, the polymer is isolated from biological material. In someembodiments, the polymer is starch, chitosan, xanthan, agar, guar gum,and the like. In other instances, e.g., wherein other polymers, such aspolyacrylonitrile (“PAN”) are optionally utilized (e.g., using DMF as asolvent).

In some instances, the nanoparticle to polymer weight ratio is at least1:5, at least 1:4, at least 1:3, at least 1:2, at least 1:1, at least2:1, or the like. In some embodiments, the monomeric residue (i.e.,repeat unit) concentration of the polymer in the fluid stock is at least100 mM. In specific embodiments, the monomeric residue (i.e., repeatunit) concentration of the polymer in the fluid stock is at least 200mM. In more specific embodiments, themonomeric residue (i.e., repeatunit) concentration of the polymer in the fluid stock is at least 400mM. In still more specific embodiments, the monomeric residue (i.e.,repeat unit) concentration of the polymer in the fluid stock is at least500 mM. In some embodiments, the fluid stock comprises at least about0.5 weight %, at least about 1 weight %, at least about 2 weight %, atleast about 5 weight %, at least about 10 weight %, or at least about 20weight polymer.

Some embodiments suitable for producing pure PAN and PAN/TiO2 hybridsolutions are shown in Table 1. As shown here, different volume percentsof TiO₂ nanoparticles (e.g., PAN/TiO₂=97:3, 92.5:7.5, 87:13 v/v %) aredispersed in 2 mL of DMF with a vortex. In some embodiments, PANpolymers are dissolved in DMF at 100° C. for 4 hours and each TiO₂/DMFsolution is then added to the PAN/DMF solution to give overall 10 wt %PAN in DMF. In some embodiments, the solutions are then electrospun ontoa metal plate covered with aluminum foil. In some embodiments, pure PANsolution and PAN/TiO₂ (7.5 v %) solutions are electrospun onto 3″×3″commercial cellulose filters (bare filter) in various fiber masses forthe nanofibrous filter media. In some instances, bare filters are coatedwith nanofibers from the pure PAN solution and other bare filters withnanofibers from the PAN/TiO₂ hybrid solution, in fiber masses of 0.1,0.25 and 0.5 g/ft² for example. In some embodiments, electrospinning isperformed using an 18 gauge needle with a flow rate of 0.025 mL/min. Thepotential difference applied between the needle and collector plate is15 kV, and the distance between the plate and the tip of the needle is13 cm in some instances.

TABLE 1 Exemplary fluid stock recipes for the electrospun nanofibers.Sample name Volume ratio (%) (volume % of TiO₂) PAN TiO₂ Pure PAN 100 —PAN/TiO₂ (3.0 v %) 97.0 3.0 PAN/TiO₂ (7.5 v %) 92.5 7.5 PAN/TiO₂ (13 v%) 87.0 13.0Nanofiber Characteristics

As described herein, the nanofibers comprise charged and/or chargeablenanoparticles in some instances. For example, as shown by the SEM imagesin FIG. 2A-2D, PAN/TiO₂ (13 vol %) fibers (FIG. 2D) have a plurality ofaggregates on the surface and PAN/TiO₂ (7.5 vol %) fibers (FIG. 2C) havesome beads. Two other exemplary fibers (FIGS. 2A and 2B) have no beadson their fiber surface. In the embodiment depicted herein, the averagediameters of all the as-spun fibers range from 0.2 μm to 0.4 μm. In someembodiments, the diameter of the electrospun fibers is slightly reducedwith the increase of TiO₂ nanoparticles in the fibers, as shown in FIG.3 . Without limitation, it is thought that the diameter of the fiberswith high contents of TiO₂ (13 vol %) embodied herein is thicker thansome embodiments with lower contents of particles because of impairedspinnability due to some aggregates of the particles. In someembodiments, the concentration of nanoparticles is suitably low suchthat the fluid stock has desirable properties for electrospinning.

In some embodiments, the presence and uniform distribution ofnanoparticles (e.g., TiO₂) in the fibers is significant for filterperformance of nanofibrous filter media as a functional medium. In someembodiments, the incorporation of TiO₂ particles in the as-spun fibersand their influence on PAN crystallization is measured by XRD. In someinstances, XRD measures the crystalline patterns of both of TiO₂particles and the all as-spun fibers. As can be seen in the embodimentdepicted in FIG. 4 , the XRD spectra show that the particles revealedtwo peaks at 25.2° and 37.8°, and the pure PAN exhibited a dominantcrystal peak at around 17°. With increasing TiO₂ content in the fibers,the peaks at 25.2° and 37.8° became gradually sharper and the peak at17° disappeared completely with the PAN/TiO₂ (13 vol %) hybrid fibers,showing that the particles were embedded in the PAN fibers. In thisembodiment, high contents of TiO₂ particles (13 v %) in the fibersdisrupted PAN crystals. In some embodiments, the concentration ofnanoparticles is such that polymer crystals are not disrupted.

The nanofibers described herein are incorporated into filter media insome instances. In the following example, pure PAN fibers and PAN/TiO₂(7.5 vol %) hybrid fibers are used to fabricate nanofibrous filtermedia. Here, the presence of TiO₂ particles in the PAN/TiO₂ (7.5 vol %)hybrid fibers is evaluated using the EMPA technique as shown in FIG. 5 .Here, the image of FIG. 5A was captured by EMPA to match the same spotwith the color coded image. The image of FIG. 5B shows the color codedimage from EMPA, which indicates the presence of Ti throughout thefibers. FIGS. 5A-B confirm that the TiO₂ particles are well distributedin the electrospun fibers in this instance.

In one aspect, the nanofibers described herein are charged and/orcomprise a plurality of regions of high charge density. In FIG. 6 , theTSC spectra of exemplary as-spun fibers show the currents generatedwhile the fibers are being heated at a ramping temperature of 7° C./min,which is observed at room temperature. Here, to characterize the currenttrends according to the contents of TiO₂ particles in the PAN fibers andthe PAN/TiO₂ hybrid fibers, four electrospun fiber samples were preparedas listed in Table 1. The fibers with high contents of TiO₂ particles(7.5 and 13 vol %) show an increase in positive charge at thetemperature 20° C. and then a drop to negative charge at 22° C. In thisembodiment, pure PAN fibers and PAN/TiO₂ (3.5 vol %) hybrid fibers showsmall changes in charge in the temperature range of 18 to 23° C. In thisembodiment, the fibers with high contents of TiO₂ particles (7.5 and 13vol %) show slightly higher current around 25° C. FIG. 7 shows exemplarytime-dependent TSC spectra of the current changes with the preparedsamples and the blank (no sample), which were monitored at 19° C. for 10min With the increase of TiO₂ particle contents in the hybrid fibers,the measured currents became higher in this embodiment. Withoutlimitation it is believed that the observed currents are the result ofthe release of charges that were displaced within the samples. Byheating a material, a displacement current is observed when atemperature is reached at which the charges are mobile. In someembodiments, the content of TiO₂ particles in the hybrid fibersinfluences the charge of the fibers and/or the filtration efficiency ofthe nanofibrous filter media.

The concentration of nanoparticles in the nanofibers is adjusted in anysuitable manner. In one example, the nanofibrous filter media areprepared with the PAN/TiO₂ (7.5 vol %) hybrid fibers. In someembodiments, 13 vol % TiO₂ in the hybrid fibers induces the deformationof fiber crystal structures, leading eventually to the change ofphysical properties of PAN fibers, and a plurality of aggregates on thesurface of fibers.

FIG. 15 illustrates a plot of zeta potential in mV versus pH for PANfibers with and without TiO₂ nanoparticles. In some embodiments, afilter comprising nanofibers described herein has a zeta potential of atleast 0 mV at a pH of less than 7. In some specific embodiments, afilter comprising nanofibers described herein has a zeta potential of atleast 0 mV (e.g., at least 10 mV) at a pH of 6. In certain specificembodiments, a filter comprising nanofibers described herein has a zetapotential of at least 0 mV (e.g., at least 10 mV) at a pH of 5.

Filter Cartridges and other Applications

The charged nanofibers (and/or compositions including nanofibers)described herein are incorporated or capable of being incorporated intoany suitable device, product, process, and the like. For example, thepresent invention encompasses a battery, capacitor, electrode, solarcell, catalyst, adsorber, filter, membrane, sensor, fabric, and/ortissue regeneration matrix comprising the nanofibers described herein.Also included are methods for making a battery, capacitor, electrode,solar cell, catalyst, adsorber, filter, membrane, sensor, fabric, and/ortissue regeneration matrix comprising the ordered porous nanofibersdescribed herein. For example, the charged nanofibers described hereincan be incorporated into the filter cartridges as described in U.S.Provisional Patent Application 61/538,458 filed on Sep. 23, 2011, whichis incorporated by reference herein for such disclosure.

Any filter comprising the nanofibers described herein is encompassed bythe present invention. In some embodiments, the filters described hereinremove particles based on at least two modes. Exemplary filtration modescomprise filtration by selective retention of charged particles,selective retention of large particles, and selective retention ofhydrophobic particles. In some embodiments, described herein are filterssuitable for selectively retaining charged particulates, largeparticulates and hydrophobic particulates.

In one aspect, the filters described herein achieve efficient filtrationat a low pressure drop. In one embodiment, described herein is a filtercomprising nanofibers, wherein the filter is suitable for retaining atleast 95% of fluid bound particulates (e.g., particle size between 0.1and 0.5 μm or between about 1 and 30 nm) challenged against the filter,wherein the pressure drop across the filter medium is at most 4 PSI atan air velocity of 80 cm/s.

In some embodiments, the nanofibers are disposed on a substrate. In someembodiments, the filters are non-woven. The substrate can be anysuitable material, and optionally provides support for the nanofibers.Some filtration may take place in the substrate. The filter has anysuitable density of nanofibers. In some embodiments, the density of thenanofibers comprising the filter is about 0.1 g/ft², about 0.25 g/ft²,about 0.5 g/ft², about 1 g/ft², about 5 g/ft², and the like. In someembodiments, the density of the nanofibers comprising the filter is atleast 0.1 g/ft², at least 0.25 g/ft², at least 0.5 g/ft², at least 1g/ft², at least 5 g/ft², and the like. In some embodiments, the densityof the nanofibers comprising the filter is at most 0.1 g/ft², at most0.25 g/ft², at most 0.5 g/ft², at most 1 g/ft², at most 5 g/ft², and thelike.

In some embodiments, the filter is configured to filter air in a heatingventilation and air conditioning (HVAC) system. In some embodiments, thefilter is suitable for capturing viruses, microbial organisms (e.g.bacteria), chemicals (e.g. pesticides), or any combination thereof.

Nanofibrous Filter Media Characteristics

The filter media described herein are porous and/or comprise a pluralityof ordered pores in some instances. As shown in FIG. 8 , therelationship of pore size distribution versus pore diameter is measuredby a Capillary Flow Porometry (CFP) in some instances. In someembodiments, the pore size distribution and the pore diameter depend onthe fiber coverage density and fiber size on a bare filter for example.As the mass of the spun fibers on the bare filter increases, the poresize distribution becomes narrower and the pore diameter smaller in someinstances. For example, the filter media with the pure PAN fibers (0.1g/ft²) and the PAN/TiO₂ hybrid fibers (0.1 g/ft²) show a pore sizedistribution and pore sizes ranging from 0.6 to 3 μm and 1.0 to 10 μm,respectively. In this instance, the highest mass of the pure PAN fiberson the bare filter had the narrowest pore size distribution and thesmallest pore diameter among the prepared samples, with a pore diameterof 0.4-0.5 μm. Here, though the PAN/TiO₂ hybrid fibers were spun on thebare filter with the same weight as the pure PAN fibers, the fibercoverage density of the spun hybrid fibers on the bare filter was lowerbecause the density of TiO₂ (4.23 g/cm³) is higher than that of PAN(1.18 g/cm³). In this instance, the PAN/TiO₂ (7 vol %) hybrid fiberscontain 23 wt % of TiO₂ particles in the PAN fibers and weighapproximately 160% of the pure PAN fibers. In some embodiments, the poresize distribution and the pore diameter are highly related to the weightof spun fibers on the bare filter. In some embodiments, these poreparameters influence the filtration efficiency of the nanofibrous filtermedia.

In one aspect, the filter media described herein have a low pressuredrop. In the CFP method, pressure drop and volume air flow through thesample are measured, optionally in both dry and wet conditions. In thedepicted embodiment, the pressure drop increases with the increase ofair flow for the dry samples, as shown in FIG. 9 . In this instance, thebare filter exhibited a pressure drop of 0.43 PSI at the air flow of 20L/m. The addition of PAN nanofibers on the bare filter increases thepressure drop, and the increase in pressure drop becomes more prominentwith increasing coverage of nanofibers. In some instances, the pure PANnanofibrous filter media show higher pressure drop with the increase ofair flow than the counterpart PAN/TiO₂ hybrid nanofibrous filter media.Here, even though the same nanofiber masses between the pure PAN and thePAN/TiO₂ hybrid were covered on the substrate, the density of thePAN/TiO₂ hybrid fibers is much higher than pure PAN fibers. In thisexample, the fiber coverage density on the substrate is considerablydifferent between the two fiber systems and they induce differentpressure drops. At the air velocity of 80 cm/s during the filtrationtest which corresponds with the air flow of around 16 L/min at CFP (tubediameter of CFP is around 2.1 cm), the pressure drop of the filter mediacovered with the PAN/TiO₂ hybrid nanofibers (0.5 g/ft²) is approximately4 times less than the counterpart filter media in some embodiments.

In some embodiments, the pressure drop of a filter provided hereinhaving nanofibers deposited on a filter substrate (e.g., base cellulosefilter) is less than 1.2 times, less than 1.5 times, less than 2 times,less than 5 times, less than 10 times that of the filter substrate(e.g., base cellulose filter) without the nanofibers, e.g., at an airvelocity of 20 cm/s, 40 cm/s, 60 cm/s, 80 cm/s, 100 cm/s. In someembodiments, the pressure drop of a filter provided herein havingnanofibers deposited on a filter substrate (e.g., base cellulose filter)is less than 2 PSI, less than 4 PSI, or the like that of the filtersubstrate (e.g., base cellulose filter) without the nanofibers, e.g., atan air velocity of 20 cm/s, 40 cm/s, 60 cm/s, 80 cm/s, 100 cm/s. Forexample, FIG. 16 illustrates the improved pressure drop characteristicsof nanofibers comprising nanoparticles described herein over nanofiberswithout the nanoparticles.

Filtration Characteristics

In one aspect, described herein are filters comprising the nanofibersdescribed herein. In some embodiments, the filter is suitable forselectively retaining charged particles, large particles and hydrophobicparticles. In one embodiment, SEM images of the PAN nanofibrous filtermedia and the PAN/TiO₂ hybrid nanofibrous filter media are taken beforeand after filtration testing as shown in FIG. 10A-10D. Images of the PANnanofibers (0.1 g/ft² and

0.5 g/ft²) on the bare filter are shown before and after the filtrationtest in FIGS. 10A-B and 10C-D, respectively. Images of the PAN/TiO₂hybrid nanofibers (0.1 g/ft² and 0.5 g/ft²) before and after thefiltration test are also taken in FIGS. 10E-F and G-H, respectively. Theimages taken after the filtration test show the trapped KCl particles onthe filters. In this embodiment, a difference of the fiber densitybetween the prepared filter media covered with the pure fiber and thehybrid fiber, 0.1 g/ft² and 0.5 g/ft², is observed by comparing images10A and 10E or 10C and 10G. The filter media made with the hybrid fibershave a lower fiber density. Here, the filter media were also observedafter the filtration test to evaluate the particle deposition on thefilter media. In this instance, the filter media covered with thehighest weight (0.5 g/ft²) of spun fibers showed higher density ofparticle deposition than the ones with low weight (0.1 g/ft²). In thisinstance, there is not a striking visual difference in the amount ofparticle deposition on the pure PAN system and the hybrid system filtermedia. In this embodiment, the electrospun nanofibers covering the barefilter captured a wide range of particle sizes on their surface. Some ofthe particles stuck to the nanofibers were 100 to 150 nm in size. Insome embodiments, the fibers are not broken during the filtration test,even with high air velocity.

In one embodiment, six nanofibrous filter specimens are fabricated forthe filtration test, with the different fiber masses (0.1, 0.25, and 0.5g/ft²) of the pure PAN fibers and the PAN/TiO₂ hybrid fibers. Here, themass of the spun fibers on the cellulose substrate is directly weighedand related to electrospinning time. Particles with diameters rangingfrom 0.1 μm to 2.0 μm are generated for the filtration test usingpotassium chloride (KCl) solution. These generated particles are mixedwith air and passed through a filter specimen at a surface velocity of80 cm/s. In some embodiments, to calculate the filtration efficiency ofthe filter specimen, the particles with 0.1-2.0 μm in diameter in theair going through the filter specimen (upstream) as well as in the aircoming out after filtering (downstream) are counted using a laserparticle counter. In FIG. 11 , an example of typical particle count dataobtained at the upstream and downstream measurement points during afiltration efficiency test is shown. The filtration efficiency (FE) iscalculated based on Equation 1:

$\begin{matrix}{{F\; E} = {\left( {1 - \frac{{average}\mspace{14mu}{downstream}\mspace{14mu}{particle}\mspace{14mu}{count}}{{average}\mspace{14mu}{upstream}\mspace{14mu}{partilce}\mspace{14mu}{count}}} \right) \times 100}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

For all six filter specimens tested in this instance, the filtrationefficiency was computed in seven particle size ranges: 0.1-0.2 μm,0.2-0.3 μm, 0.3-0.4 μm, 0.4-0.5 μm, 0.5-0.7 μm, and 0.7-1.0 μm as wellas an average over the range 0.1-1.0 μm. Exemplary filtrationefficiencies measured are reported in Table 2. It can be observed thatthere is a general improvement in filtration efficiency with theincrease of the fiber coverage covering the bare filter in the presentembodiment. In addition, the PAN/TiO₂ hybrid filter specimens showedhigher filtration efficiencies than their pure PAN counterparts.

TABLE 2 Filtration efficiencies of the nanofiber filter media accordingto the particle size. Sample name (fiber coverage Particle size (μm)density (g/ft²)) 0.1-0.2 0.2-0.3 0.3-0.4 0.4-0.5 0.5-0.7 0.7-1.0Avg0.1-1.0 PAN (0.1) 89.9 92.6 93.4 93.6 93.4 91.2 91.1 PAN (0.25) 87.791.4 93.5 95.5 97.2 98.7 89.5 PAN (0.5) 92.4 94.5 95.3 95.8 96.3 97.093.4 PAN/TiO₂ (0.1) 86.7 92.4 95.1 96.7 97.6 98.3 90.0 PAN/TiO₂ (0.25)95.4 96.9 97.4 98.2 98.6 99.0 96.2 PAN/TiO₂ (0.5) 98.4 99.1 99.3 99.399.3 99.1 98.8

In FIG. 12 , the filtration efficiencies in the particle ranges 0.1-0.5μm for the pure PAN and PAN/TiO₂ hybrid nanofibrous filter specimens areplotted together. Here, with increasing the particle size, the allfilter specimens showed increasing filtration efficiency. In thisinstance, the pure PAN nanofibrous filter media with 0.1 g/ft² fibermass showed slightly higher filtration efficiency up to particle sizesof 0.4 μm than the one with 0.25 g/ft² fiber mass. In some embodiments,this is because the pure PAN nanofibrous filter media with 0.1 and 0.25g/ft² fiber masses do not have the capability to filter out the smallparticle sizes (below 0.4 μm) because of low fiber coverage density onthe bare filter. In the case of very small particle sizes, thefiltration efficiency of the filter media is influenced by the locallyuneven fiber density on the bare filter in some instances. In someembodiments, locally higher or lower fiber coverage density on the barefilter is fabricated due to the short electrospinning time, especiallywith the 0.1 g/ft² fiber coverage density. In some instances with smallparticle size, the filtration efficiency of the PAN/TiO₂ (0.1 g/ft²)hybrid nanofibrous filter specimen showed the lowest filtrationefficiency, but the PAN/TiO₂ (0.25 and 0.5 g/ft²) resulted in higherfiltration efficiency than the pure PAN system, even though the hybridsystem had a larger pore diameter than the pure system. In particular,the PAN/TiO₂ (0.5 g/ft²) nanofibrous filter media show high filtrationefficiency (over 98%) even with the smallest particle sizes (0.1-0.2 μm)and over 99% filtration results with larger particle sizes than 0.3 μmin some embodiments. In some embodiments the filters described hereinhave a long operating life. For example, without limitation, if thePAN/TiO₂ nanofibrous filter media have the same fiber mass of 1.0 g/ft²as the nylon 6 nanofibrous filter media, the PAN/TiO₂ nanofibrous filtermedia have less fiber coverage on the bare filter so that they areexpected to have much longer operating time due to the low pressure dropand show superior filtration efficiency based on the filtration resultsfrom the PAN/TiO2 (0.5 g/ft²) nanofibrous filter media. In someembodiments, compared to the pure PAN system (0.5 g/ft²), the PAN/TiO₂hybrid system results in better filtration efficiency (6%) with thefinest particle size (0.1-0.2 μm) even though the hybrid system hadlarger pores in the filter media.

In some instances, overall filtration efficiency depends at least inpart on the fiber coverage density on the bare filter and the presenceof TiO₂ particles in the fibers. The mean pore diameter (measured byCFP) and the overall average filtration efficiency, which was measuredwith the particle sizes of 0.1 to 1.0 μm, are plotted in FIG. 13 , alongwith the fiber coverage density for one embodiment. In this embodiment,the pure PAN nanofibrous filters show smaller pore diameter than theircounterparts with PAN/TiO₂ hybrid nanofibers. For example, in thehighest fiber coverage density (0.5 g/ft²), two types of filter mediahave a similar mean pore diameter that is small (below 1.0 μm), whereasfilters with lower fiber coverage densities (0.1 and 0.25 g/ft²) show alarge difference in the mean pore diameter. In this embodiment, thefiltration efficiency of the PAN/TiO₂ hybrid nanofibrous filter media ishigher than the pure PAN system except for the lowest fiber coveragedensity (0.1 g/ft²), even though the PAN/TiO₂ hybrid nanofibrous filtershave a larger pore diameter. In the case of the 0.1 g/ft² fiber coveragedensity, the mean pore sizes of the pure system and the hybrid systemare approximately 1.5 and 4.8 μm, respectively, indicating that the puresystem has smaller pores in this embodiment. Here, the overallfiltration efficiency of particles with size 0.1-1.0 μm shows verysimilar results. In the case of the 0.25 g/ft² and 0.5 g/ft² fibercoverage densities, the hybrid system shows higher filtration efficiencythan the pure system. The nanoparticles (e.g., TiO₂) of the hybridfibers are one factor that affects the filtration efficiency in someinstances. For example, in an embodiment presented herein the filtrationability of the hybrid nanofibrous filter media (comprisingnanoparticles) is considerably higher than the pure PAN system, eventhough in this instance the fiber coverage density of the pure PANfilter is 0.5 g/ft² and only 0.25 g/ft² for the hybrid media. If it werenot for the effect of TiO₂ particles attracting the KCl particles, thefiltration efficiency would have sharply decreased with the addition ofthe TiO₂ particles to the fibers in this instance because the hybridnanofibrous filter media have larger pore sizes than their pure PANcounterparts. Thus, in some instances, the fiber coverage density andthe presence of TiO₂ particles in the nanofibers are two factors in thecontrol of filtration efficiency. It should be noted that as the TiO₂particles were incorporated into the fibers, the fiber coverage densitydecreased, compared with the pure PAN fibers. In some instances, TiO₂particles in the PAN/TiO₂ hybrid fibers contributed to enhancefiltration efficiency with much less pressure drop.

In one aspect, the pore properties of the filters are considered afactor that can influence the filtration efficiency. Filters withsmaller pore sizes are generally expected to have higher filtrationefficiency. As described herein, CFP is used to characterize the PAN andPAN/TiO₂ nanofibrous filter media in some instances. The PSD shiftedtowards smaller pore diameters when the fiber coverage density wasincreased from 0.1 g/ft² to 0.5 g/ft² for both the filters covered withthe PAN fibers and the filters covered with PAN/TiO₂ fibers in someembodiments. When the PSD was compared for the filter media having equalfiber coverage density but differing in the presence or absence of TiO₂nanoparticles, it was found that the filter media with PAN/TiO₂ hybridnanofibers had a larger average pore size than the pure PAN filters insome embodiments. In one aspect, the filtration efficiency is expectedto be greater in the PAN nanofibrous filters due to their smaller poresizes. However, as described in some embodiments herein, PAN/TiO₂ hybridnanofibrous filters are notably more effective at filtering smaller,harder-to-catch particles. For some embodiments having high fibercoverage density (e.g., 0.5 g/ft²), the filtration has a high efficiency(over 98%), regardless of particle size. Without limitation, onepossible reason for the superior filtration efficiency of PAN/TiO₂hybrid filter is electrostatic interactions between the nanoparticles(e.g., TiO₂) and the particles to be filtered (e.g., KCl).

EXAMPLES Example 1—Materials

Polyacrylonitrile (PAN) (Mw 150,000) and Dimethylformamide (DMF) werepurchased from Sigma-Aldrich Co. The anatase TiO2 with a particle sizeranging 5-10 nm was supplied by Samsung Co. (Korea). The cellulose barefilter media used in the examples was provided by Clark Filter Inc.(USA). The fiber diameters of the bare filter ranged between 20 μm and50 μm.

Example 2—Fabrication of Nanofibers and Nanofibrous Filter Media

Pure PAN solution and three PAN/TiO2 hybrid solutions were prepared asshown in Table 3. First, three different volume percents of TiO2nanoparticles (PAN/TiO2=97:3, 92.5:7.5, 87:13 v/v %) were dispersed in 2mL of DMF with a vortex, respectively. PAN polymers were dissolved inDMF at 100° C. for 4 hours and each TiO2/DMF solution was then added tothe PAN/DMF solution to give overall 10 wt % PAN in DMF. The solutionswere then electrospun onto a metal plate covered with aluminum foil sothat four nanofiber mats were prepared for characterization. Pure PANsolution and PAN/TiO2 (7.5 v %) solution were electrospun onto 3″×3″commercial cellulose filters (bare filter) in various fiber masses forthe nanofibrous filter media. Three bare filters were coated withnanofibers from the pure PAN solution and another three bare filterswith nanofibers from the PAN/TiO₂ hybrid solution, in fiber masses of0.1, 0.25 and 0.5 g/ft². Electrospinning was performed using an 18 gaugeneedle with a flow rate of 0.025 mL/min. The potential differenceapplied between the needle and collector plate was 15 kV, and thedistance between the plate and the tip of the needle was 13 cm.

TABLE 3 Exemplary fluid stock recipes for the electrospun nanofibers.Sample name Volume ratio (%) (volume % of TiO₂) PAN TiO₂ Pure PAN 100 —PAN/TiO₂ (3.0 v %) 97.0 3.0 PAN/TiO₂ (7.5 v %) 92.5 7.5 PAN/TiO₂ (13 v%) 87.0 13.0

Example 3—Characterization of Nanofibers and Nanofibrous Filter Media bySEM

The morphology of the electrospun fibers and the surface of thenanofibrous membrane before and after filtration testing were examinedwith a Leica 440 scanning electron microscope (SEM) after being coatedwith Au—Pd. The diameter of the spun fibers was measured using Imageanalysis software (ImageJ 1.41).

Example 4—Characterization of Nanofibers and Nanofibrous Filter Media byElectron Micropore Analyzer (EMPA)

The presence of TiO2 nanoparticles in the as-spun fibers was confirmedusing an electron microprobe analyzer (EMPA, Jeol Model 8900R). EMPA wasused in the wavelength dispersive spectrometer mode for TiO2 mapping. Asthe TiO2 particles are present in the PAN/TiO2 hybrid fibers, the TiO2map directly indicates the location of the TiO2 particles in the as-spunfibers.

Example 5—X-Ray Diffraction Measurement (XRD)

XRD measurements were performed to confirm the presence of TiO2nanoparticles in the as-spun fibers and their influence on PANcrystallization by a Scintag Theta-theta X-ray Diffractometer(nickel-filtered CuKα radiation, λ=1.54 A° operating at 45 kV and 44 mA.All data were collected in the 2θ range of 5-45° with a step of 0.03°and a scanning rate of 5°/min.

Example 6—Thermally Stimulated Current Measurements (TSC)

The electrical charge potential of the as-spun fibers was measured by aTSC instrument (Novocontrol tech. GmbH & Co., Germany) Two TSC methodswere adopted to probe the current behaviors of the as-spun fibers. Inthe first method, the measurement was performed at the rampingtemperature 7° C./min from 10° C. to 30° C. in the absence of anexternally applied voltage. In the second method, the surface currentcharges of the as-spun fibers were evaluated without applied voltage sothat TSC spectra at a fixed temperature (19° C.) were recorded for 10min During the measurement, the specimen of the as-spun fibers wassandwiched between two pieces of 20 μm Teflon film to prevent thecontact between the two electrodes. The thickness of as-spun fibers wasapproximately 20 μm.

Example 7—Capillary Flow Porometry

A capillary flow porometry (Porous Media Inc., NY) was used tocharacterize the pore size and pore size distribution of the fibrousfilter media. The fibrous filter media were cut into a diameter of 2.5cm for porometry measurement. In this method, pores of the filter mediawere filled with a few drops of a wetting liquid (SilWick fluid, surfacetension 20.1 dyne/cm). The liquid in the pores (wet sample) was removedby pressure-driven air flowing. The larger pores were emptied at thelowest pressure and smaller pores were gradually emptied with increasingpressure. The air flow was maximized when the sample was dry. Thedifference in air pressures and air flow rates between the wet and drysample were measured. The relationship between pressure and flow ratefor the dry and wet sample was used to calculate the mean pore diameterand the pore size distribution (PSD).

Example 8—Filtration Test

The filtration test of nanofibrous filter media was performed with amulti-channel filtration test device (at the Environmental SystemsLaboratory of Syracuse University, NY). As can be seen in FIG. 1 , thedevice consists of five 2″ diameter tubes (or channels) that separatelyhold each filter specimen with diameter of 2 inches. The device isattached to a particle generator which generates KCl particles of0.1-2.0 μm in diameter (not seen in FIG. 1 ). The mixing box at thebottom of the device creates a substantially uniform distribution of thegenerated KCl particles into each upstream channel. An air stream withKCl particles is passed through the filter at a surface air velocity of80 cm/s. A particle counter is used to measure particle concentration,first upstream of the filter for 3 minutes and then downstream of thefilter for another 3 minutes. In this example, the six nanofibrousfilter media were subjected to a filtration efficiency test.

Example 9—Nanoparticle in Polymer Matrix—Nanofibers (with GasAssistance)

0.5 grams of preformed nanoparticles (100 nm average diameter), themetal component, is suspended in 20 ml of 1 molar acetic acid solutionwith X-100 surfactant. The solution is stirred for 2 hours to create asuspension of nanoparticles.

In a second solution, 1 gram of 99.7% hydrolyzed polyvinyl alcohol (PVA)with an average molecular weight of 79 kDa and polydispersity index of1.5 is dissolved in 10 ml of de-ionized water. The polymer solution isheated to a temperature of 95° C. and stirred for 2 hours to create ahomogenous solution.

The nanoparticle suspension is then combined with the PVA solution tocreate a fluid stock. In order to distribute the nanoparticlessubstantially evenly in the fluid stock, the nanoparticle suspension isadded gradually to the polymer solution while being continuouslyvigorously stirred for 2 hours. The mass ratio of nanoparticles topolymer for the fluid feed is 1:4.

The fluid stock is co-axially electrospun with gas using a coaxialneedle apparatus similar to the one depicted in FIG. 19 . The centerconduit contains fluid stock and the outer conduit contains air. Theelectrospun hybrid fluid stock (hybrid as-spun nanofiber) is optionallyheat treated for 2 hours at 600° C. in an inert atmosphere (e.g.,argon). FIG. 18 illustrates a TEM image of such a nanofiber and thenon-aggregated nanoparticles therein.

Example 10—Nanoparticle in Polymer Matrix—Nanofibers (without GasAssistance)

Using a procedure similar to that set forth in Example, nanocompositenanofibers comprising nanoparticles were prepared without gasassistance. FIGS. 17A-D illustrate various TEM images of such(non-thermally treated) nanofibers.

What is claimed is:
 1. A filter comprising: a nanofiber comprising afirst material comprising a polymer and a second material comprising acharged or chargeable material; wherein the charged or chargeablematerial constitutes or comprises one or more regions of high chargedensity on or in the nanofiber configured to capture at least part of aplurality of particles in a fluid against the filter, wherein thenanofiber comprises highly charged domains and non- or low-chargedmatrix wherein the highly charged domains have an average charge perunit size of at least 2 times of the average charge of the non- orlow-charged matrix.
 2. The filter of claim 1, wherein the nanofiber hasregions of positive charge and regions of negative charge, and thenanofiber is neutrally charged overall.
 3. The filter of claim 1,wherein the charged or chargeable material comprises one or more chargedor chargeable nanoparticles.
 4. The filter of claim 1, wherein the fluidis air or a liquid and wherein the particles in the fluid comprisesviruses, microbial organisms, chemicals, or any combination thereof. 5.The filter of claim 1, wherein the filter retains at least 95% offluid-bound particles challenged against the filter.
 6. The filter ofclaim 5, wherein the particles have a size between 0.1 microns and 0.5microns or between about 1 nm and 30 nm.
 7. The filter of claim 1,wherein a pressure drop across the filter is at most 4 PSI at an airvelocity of 80 cm/s.
 8. The filter of claim 1, further comprises asubstrate and wherein the substrate comprises fibers thicker than thenanofiber.
 9. The fiber of claim 1, wherein the charged or chargeablematerial forms a plurality of discrete regions of high charge densityembedded within the nanofiber.
 10. The fiber of claim 1, wherein thenanofiber comprises a coating comprising the charged or chargeablematerial.
 11. The fiber of claim 1, wherein the charged or chargeablematerial is deposited directly on a substrate.
 12. The filter of claim1, wherein the nanofiber has a non-zero net charge without appliedvoltage.
 13. The filter of claim 1, wherein a plurality of charged orchargeable nanoparticles are uniformly distributed in the nanofiberwithout aggregation.
 14. The filter of claim 1, wherein the nanofiberhas a non-homogeneous charge density.
 15. A method of making a filter ofclaim 1 comprising: providing a fluid stock comprising a polymer and acharged or chargeable material; electrospinning the fluid stock onto asubstrate to form a plurality of nanofibers directly deposited on thesubstrate.
 16. A filter comprising: a nanofiber comprising a firstmaterial comprising a polymer and a second material comprising a chargedor chargeable material; wherein the charged or chargeable materialconstitutes or comprises one or more regions of high charge density onor in the nanofiber configured to capture at least part of a pluralityof particles in a fluid against the filter, wherein a standard deviationof a charge density of the nanofiber is at least 50%.
 17. A filter ofclaim 16, wherein the filter is configured to retain at least 95% offluid-bound particles challenged against the filter and wherein thefluid is air or a liquid, and the particles in the fluid comprisesviruses, microbial organisms, chemicals, or any combination thereof. 18.The filter of claim 16, wherein the nanofiber has regions of positivecharge and regions of negative charge, and the nanofiber is neutrallycharged overall.
 19. A filter comprising: a nanofiber comprising a firstmaterial comprising a polymer and a second material comprising a chargedor chargeable material; wherein the charged or chargeable materialconstitutes or comprises one or more regions of high charge density onor in the nanofiber configured to capture at least part of a pluralityof particles in a fluid against the filter, wherein the nanofibercomprises a plurality of regions having high charge density and whereinthe average charge density of the regions having high charge density isat least 200% of the average net charge density of the nanofiber.
 20. Afilter of claim 19, wherein the filter is configured to retain at least95% of fluid-bound particles challenged against the filter and whereinthe fluid is air or a liquid, and the particles in the fluid comprisesviruses, microbial organisms, chemicals, or any combination thereof. 21.The filter of claim 19, wherein the nanofiber has regions of positivecharge and regions of negative charge, and the nanofiber is neutrallycharged overall.
 22. A filter comprising: a nanofiber comprising a firstmaterial comprising a polymer and a second material comprising a chargedor chargeable material; wherein the charged or chargeable materialconstitutes or comprises one or more regions of high charge density onor in the nanofiber configured to capture at least part of a pluralityof particles in a fluid against the filter, wherein the nanofibercomprises a plurality of regions having high charge density and whereinthe regions comprising high charge density are separated by at least 100nm on average.
 23. A filter of claim 22, wherein the filter isconfigured to retain at least 95% of fluid-bound particles challengedagainst the filter and wherein the fluid is air or a liquid, and theparticles in the fluid comprises viruses, microbial organisms,chemicals, or any combination thereof.
 24. The filter of claim 22,wherein the nanofiber has regions of positive charge and regions ofnegative charge, and the nanofiber is neutrally charged overall.